In other words, sex is not unlike sports. If you want to be a good tennis player, play lots of tennis; if you want to be a good lover, make lots of love.

This maxim springs more from anecdotal observations than from scientific studies: Men who have erection problems tend not to have much sex, urologists noticed. And those who don't, have plenty. Then again, anyone with a passing knowledge of the birds and the bees might have guessed as much.

So, which happens first in the use-it-or-lose-it Olympics? In sports as in sex, we like to do what we're good at doing. Perhaps when some men find that their previous prowess -- be it at backhands or erections -- isn't what it used to be, they start sitting out more and more games.

A new study following nearly 1,000 Finns, published July in the American Journal of Medicine, might help answer this question.

Researchers with the Tampere Male Urological Study in Finland first sent questionnaires to men ages 50 to 75, inquiring as to the quality of their erections and the frequency of their intercourse. (The researchers didn't ask about masturbation.) Men who reported no major problems were included in the study.

Next, the researchers sat back and waited five years. Then they tracked down the men again and asked about their general health -- including any erection problems that had developed since the first questionnaire.

Results support the use-it-or-lose-it hunch. Having sex once a week halved men's chances of developing erectile dysfunction, compared with a less-than-once-a-week schedule. (The study also hinted that the more often men had sex, the better they fared.) In their analyses, the researchers made sure that other health factors -- such as heart disease, diabetes, obesity and depression -- did not explain the differences among men.

So what's going on here? Why does having more sex keep the male organ in trim instead of wearing it out? The likely elixir is oxygen, says study lead author Dr. Juha Koskimäki, consulting urologist at Tampere University Hospital -- specifically, oxygen molecules summoned to particular parts of the penis by erections.

When a man is sexually aroused -- whether by a touch to the penis or a favorite Jenna Jameson flick -- nerves stretching from the brain to the base of the spine and throughout the penis kick into action, triggering chemical and electrical signals.

More blood flows into the penis. In the two flexible tubes of spongy tissue that extend the length of the shaft (the corpora cavernosa, or "cave-like bodies"), muscle tissue slackens. Their tiny caverns swell with blood, straining against the surrounding membranes. Blood gets trapped in the organ. The result is a longer, stiffer penis -- and also, because blood brings oxygen with it, a thoroughly oxygenated one.

Penile tissue is like other tissue: Oxygen is good for it. But unlike, say, muscles in the forearm, the corpora cavernosa tissue spends no more than three hours a day -- a good day -- fully oxygenated.

Some scientists even wonder if the unconscious erections that occur during sleep might be the body's way of making sure oxygen levels in the corpora cavernosa stay high. And perhaps that's why even 12-week-old fetuses can have erections in the womb -- because periodic oxygenation is important.

Some research has suggested chronically low levels of oxygen in the corpora cavernosa tissue might reduce its flexibility and gradually turn it into a sort of scar tissue that won't respond well to chemical signals during sexual arousal.

Oxygen-deprivation might also explain why some men lose erectile functioning after prostatectomies -- and never recover it. The sudden drop in erections after surgery -- usually a temporary phenomenon -- leads to low oxygen levels, researchers hypothesize, which could make nerve and tissue trauma even worse.

If so, there might be a (nonerotic) solution to these surgical woes: extreme oxygen therapy. A study published in January in the Journal of Sexual Medicine found that rats that spent time in a hyperbaric oxygen chamber at three times atmospheric pressure -- including 90 minutes of breathing pure oxygen -- kept their erectile function better than rats that didn't get this special treatment. Before we trigger a run on the oxygen bars or doctors offering hyperbaric chamber sessions, a note of caution: More research is needed before bringing the technique to humans, the investigators say.

In any case, for most men, having natural erections should keep penis tissue well supplied -- although not every erection might fit the bill. For full effect, oxygenation probably needs to last a certain minimum time, Koskimäki says, though exactly how long is still unknown. That's part of the reason his team asked only about sexual intercourse practices. "Masturbation often happens very quickly," he notes.

Dr. Ira Sharlip, a clinical professor of urology at the University of California at San Francisco and spokesman for American Urological Assn., says that the results are intriguing but further studies need to be done on sexual intercourse and erectile dysfunction before the link becomes clear. Other factors could partially explain the findings, he notes, such as inborn powers. "Maybe some people are wired to be more sexual and also to have better erections for a longer period of time," he says.

Plus, it's notoriously difficult to assess sexuality simply by asking people to fill out a questionnaire. Men who have whopping amounts of sex (or at least report doing so) might also be more inclined to gloss over the details of their erection shortcomings.

Of course, the best way of getting at cause and effect would involve an experiment -- randomly assigning some men to a few decades of porn-star athletics, for instance, and others to a relaxing, cloistered celibacy. No one appears to be in a hurry to do this trial.

"The fact is that science doesn't know," Sharlip says. His own advice to patients: "Do as the spirit moves you and make decisions about sexuality that are comfortable for you and your partner."

"Sex is for pleasure," Koskimäki adds. "If we put it onto our calendar when we have to have sex, I think we lose the positive effect of sex on health."